The transfusion of radiolabeled red cells and platelets (PLTs) allows for the accurate determination of in vivo recovery and survival after subjecting the cells to an experimental collection, processing, or storage system. The techniques utilized in these protocols have undergone refinement over the past several decades, and these improvements have reduced interlaboratory variability and improved the accuracy of the techniques. At present, the field is evolving toward new applications of these radiolabeling techniques, including, for PLTs, the use of freshly collected autologous PLTs as the standard-setting PLTs against which the "test" PLTs can be compared. The contribution of the National Blood Foundation in supporting research in these fields is acknowledged with appreciation because its efforts have helped advance not only the science involved but also the scientists engaged in this research.